Is there a way to tell Blender NOT to load certain images during render?
This can be achieved by enabling "Use Camera Cull" in the Simplify settings in the scene tab, though it will remove entire objects that are not in the camera's view instead of just textures and will affect reflections/shadows as a result.



An alternative solution to your problem is to enable "Persistent Images". Blender will keep these large textures in memory instead of reloading them on every frame, thus reducing your render time on subsequent frames without altering the final image (shadows/reflections) in any way.